diff --git a/apps/figcraft-server/src/config.rs b/apps/figcraft-server/src/config.rs
index 3e951e87..101c7815 100644
--- a/apps/figcraft-server/src/config.rs
+++ b/apps/figcraft-server/src/config.rs
@@ -135,9 +135,23 @@ impl Config {
     /// abuse contact, or one whose clients would all count as its proxy).
     pub fn warnings(&self) -> Vec<String> {
         let mut out = Vec::new();
+        let client_max = NetLimits::default().max_server_message >> 20;
+        if usize::try_from(self.file_mb()).is_ok_and(|mb| mb >= client_max) {
+            out.push(format!(
+                "max_file_mb is {}: the app opens files of up to {} MiB, so a file that large can't be opened",
+                self.file_mb(),
+                client_max - 1
+            ));
+        }
         if !self.open_publish {
             return out;
         }
+        if self.file_mb() > 64 {
+            out.push(format!(
+                "open_publish is on with max_file_mb = {}: anyone may send messages that large, and the memory set aside for reading and queueing them grows to eight times it; keep it at 64 or less on a public server",
+                self.file_mb()
+            ));
+        }
         if self.operator_contact.trim().is_empty() {
             out.push("open_publish is on but operator_contact is empty: people can't reach you about abuse; set it (and terms_url)".into());
         }
diff --git a/apps/figcraft-server/src/public_tests.rs b/apps/figcraft-server/src/public_tests.rs
index 6403212c..978d9e8e 100644
--- a/apps/figcraft-server/src/public_tests.rs
+++ b/apps/figcraft-server/src/public_tests.rs
@@ -506,8 +506,21 @@ fn files_above_64_mib_fit_every_limit() {
         assert!(net.max_reading_total >= net.max_reading_block && net.max_queued_total >= net.max_queued_block);
         assert!(core.max_message_bytes > file, "open={open_publish}: {}", core.max_message_bytes);
     }
-    // A client takes the largest file any server can keep, or as much as a 32-bit target can address
-    // (computed in u64: `4097 << 20` in a 32-bit usize wraps to 1 MiB).
-    let client = NetLimits::default().max_server_message as u64;
-    assert!(client > 4096 << 20 || client == u64::from(u32::MAX), "{client}");
+    // A client takes such a file too, but caps what any server may make it buffer at 1 GiB.
+    assert_eq!(NetLimits::default().max_server_message, 1 << 30);
+}
+
+#[test]
+fn large_files_warn_the_operator() {
+    let public_url = Some("wss://figcraft.example.com/".into());
+    let warns = |open_publish, mb| {
+        Config { open_publish, max_file_mb: Some(mb), public_url: public_url.clone(), ..Config::default() }.warnings().join(
+            "
+",
+        )
+    };
+    assert!(!warns(true, 64).contains("max_file_mb"), "{}", warns(true, 64));
+    assert!(warns(true, 65).contains("open_publish is on with max_file_mb = 65"), "{}", warns(true, 65));
+    assert!(!warns(false, 1023).contains("max_file_mb"), "{}", warns(false, 1023));
+    assert!(warns(false, 1024).contains("can't be opened"), "{}", warns(false, 1024));
 }
diff --git a/crates/collab-net/src/limits.rs b/crates/collab-net/src/limits.rs
index 02167770..a0647fa6 100644
--- a/crates/collab-net/src/limits.rs
+++ b/crates/collab-net/src/limits.rs
@@ -12,10 +12,9 @@ pub struct NetLimits {
     /// before: its hello.
     pub max_client_message: usize,
     pub max_hello: usize,
-    /// Largest message a client accepts from the host (a welcome carries the whole document): the
-    /// largest file a `figcraft-server` can be set to keep (`max_file_mb`, at most 4096) and a margin,
-    /// so a client can open every file a server takes; on a 32-bit target (the browser) as much as
-    /// `usize` holds.
+    /// Largest message a client accepts from the host (a welcome carries the whole document): 1 GiB,
+    /// room for any file up to 1023 MiB and well past the servers' defaults, while a hostile server
+    /// (any link can name one) can't make the app or a browser tab buffer more than that.
     pub max_server_message: usize,
     /// Messages a client may send per second on average, and in a burst; beyond that it is
     /// disconnected.
@@ -64,7 +63,7 @@ impl Default for NetLimits {
             max_connections: 64,
             max_client_message: figcraft_collab::Limits::default().max_message_bytes,
             max_hello: 64 << 10,
-            max_server_message: usize::try_from(4097u64 << 20).unwrap_or(usize::MAX),
+            max_server_message: 1 << 30,
             rate: 100.0,
             burst: 400.0,
             hello_timeout: Duration::from_secs(10),
diff --git a/docs/server.md b/docs/server.md
index 55d784f4..5cbab6a8 100644
--- a/docs/server.md
+++ b/docs/server.md
@@ -282,7 +282,7 @@ are errors):
 | `data` | `figcraft-data` | The data directory |
 | `public_url` | (the listen address) | How links name the server, when people reach it another way (a reverse proxy: `wss://figcraft.example.com/`) |
 | `max_files` | 1000 | Most files kept |
-| `max_file_mb` | 64 (16 when open) | Largest file (its snapshot), in MiB; a welcome carries the whole file, so messages may be as large as a file and 1 MiB (at least 64 MiB on a private server) |
+| `max_file_mb` | 64 (16 when open) | Largest file (its snapshot), in MiB; a welcome carries the whole file, so messages may be as large as a file and 1 MiB (at least 64 MiB on a private server). The app opens files of up to 1023 MiB; above 64 with `open_publish` the server warns |
 | `max_links` | 100 | Most share links per file |
 | `max_connections` | 512 | Most open connections |
 | `max_people` | 256 | Most people in one file at once |